1. Field of the Invention
The present invention relates to a curable composition of epoxy resin and cured product thereof.
2. Description of the Prior Art
Epoxy resins have been recently used in various industrial fields such as paints, electric parts, construction materials, adhesives and shaped articles owing to their excellent physical properties.
In general, epoxy resins are not used as the resin alone, but usually used in a form of a thermosetting resin having a three dimensional structure by curing with a curing agent. There have been known many curing agents. Properties of the cured resin are affected by the curing agent used. In other words, the pot life, viscosity, compatibility, degree of dispersion, curing temperature, curing time, and degree of scorching during curing, of the mixture of epoxy resin and curing agent largely depend upon the type of the curing agent. Therefore, selection of an appropriate curing agent is very important to produce a cured epoxy resin of desired properties.
As a curing system of epoxy resins, two-part systems have been generally used which comprise an epoxy resin component such as bisphenol-A type resin and a curing agent component such as a polyamine or an acid anhydride.
Much attention has been recently devoted to one-part systems of epoxy resin composition using a latent curing agent.
The one-part system of epoxy resin composition has much commercial significance as compared to the two-part system. A one-part system serves to eliminate time consuming measuring and mixing operations at the point of ultimate consumption, danger caused by error in measuring or insufficient mixing, and waste due to mixing too much.
As latent curing agents, aliphatic dicarboxylic acid dihydrazide, dicyandiamide and their mixtures are known (U.S. Pat. Nos. 2,847,396 and 3,294,748).
Amino acid hydrazide is also reported in David Aelong: J. Appl. Polymer Sci., 13, 227 (1969). Aliphatic or aromatic dicarboxylic acid dihydrazides have generally high melting points and many of them are apt to decompose at the melting point.
Moreover, since compositions containing the above well known curing agent are apt to scorch owing to exothermic reaction upon curing, the pot life is short, homogeneous mixing is not possible in a short time, and it is difficult to shape thick articles in a homogeneously cured state.
3,9-Bis(3-aminopropyl)-2,4,8,10-tetraoxaspiro-[5,5] -undecane is also known as a curing agent to give a tough epoxy resin. However, this compound is of excessively large reactivity, short pot life and offensive odor. In order to eliminate the drawbacks, the diamine is modified with a monoepoxide and the amino groups are masked to liquefy the diamine. However, such modification can not remove the drawbacks inherent to amine curing agents, for example, undesirable crystallization during storage.